Improving Directed Releases From Nyc Reservoirs In The Upper Delaware River Basin

The Delaware River Basin (DRB) supplies water to over 13 million people in four states. Historically, users in the Basin struggled to reach agreements on how to share the water in the Delaware River. In 1954, the U.S. Supreme Court issued a Decree resolving State of New Jersey v. State of New York and City of New York and specifying how much water individual parties like New York City (NYC) or New Jersey State are allowed to divert from the DRB. It also established the position of Delaware River Master at the US Geological Survey (USGS) to administer the decision. On a day-to-day basis, the Delaware River Master models the expected flows of the Delaware River at Montague, NJ, designs the reservoir releases from the three NYC reservoirs that are needed to maintain established minimum flow objectives, directs NYC to make these releases, and compiles information needed to document these releases.

A system analysis team of USGS experts performed multiple analyses to look at the Delaware River Master directed release design process. The team determined which component improvements would result in the largest increases in flow objective compliance and provided insight into the system.

The relative contributions of five components of streamflow at the Delaware River at the USGS Montague, NJ, streamgage: (1) directed releases from NYC reservoirs, 2) conservation releases from the NYC reservoirs, 3) powerplant releases, 4) baseflow for the intervening flow between the powerplants and NYC releases and the Delaware River at Montague streamgage, and 5) runoff for the intervening flow between the powerplants and NYC releases and the Delaware River at Montague streamgage) were examined to obtain the daily fraction of the total streamflow that each component contributes to the total streamflow at the streamgage and identify those components that are large contributors and highly variable. Baseflow was observed to be a substantial contribution to the total streamflow at Montague and low baseflow conditions appear to coincide with periods of directed releases. The contribution and variability of the powerplants to the total streamflow at Montague was low (~10 percent) across all years, months, and flows. An evaluation was also conducted of baseflow and streamflow recession of the Delaware River at the USGS Montague, NJ, streamgage. The evaluation used daily streamflow data for the period 2010-2017 and hydrograph-separation and recession-curve analysis techniques provided in the USGS Groundwater Toolbox. Results of the hydrograph-separation analysis indicated that groundwater discharge is a substantial component of total streamflow at the gage. A script that reproduces the directed releases design process was developed to evaluate the sources of error in the design process for the historical period 2010-2019. The combined uncertainty in precipitation runoff and baseflow forecasts had a large effect on the annually averaged directed releases errors.